Pretreatment of solid propellant grains containing ammonium nitrate



This invention relates to solid propellant grains. In one aspect, this invention relates to a method of conditioning an ammonium nitrate-containing propellant.

Broadly, this invention relates to conditioning solid propellant compositions comprising ammonium nitrate by first cycling the propellant through a phase transition temperature of the ammonium nitrate oxidant. Any temperature range which includes a phase transition of the oxidizer can be generally used. It is preferred, when practicing our invention to cycle slowly, i.e., low temperature gradient, or to limit the temperature range to a modest range on either side of the phase transition temperature chosen. Propellants thus treated in accordance with our invention, are less likely to undergo volumetric changes which result in cracks, fissures, etc., when said propellants are subjected to extreme temperature fluctuations such as would be encountered in flight from comparative low to very high altitudes, etc.

Accordingly, one or more of the following objects Will be achieved by the practice of our invention.

It is an object of this invention to provide an improved solid propellant composition.

Another object of this invention is to provide an improved solid propellant composition comprising ammonium nitrate and a rubbery binder.

Another object of this invention is to provide an improved solid propellant composition comprising a polymeric material and ammonium nitrate.

A further object is to provide a novel method of conditioning a solid propellant comprising a rubbery binder and an oxidant by cycling the propellant through a temperature range which includes a phase transition temperature of said oxidant.

A still further object of this invention is to provide a novel method of conditioning a solid propellant comprising a polymeric material and ammonium nitrate by cycling the propellant through a temperature range which includes a phase transition temperature of the ammonium nitrate.

A yet further object of this invention is to produce an improved propellant which can be subjected to severe temperature fluctuations without excessive volumetric changes of said propellant.

Other objects, advantages, and features of this invention will be apparent to one skilled in the art upon reading this specification.

In one aspect, this invention relates to pretreating an ammonium nitrate-containing propellant composition and to the conditioned propellant composition. The pretreatmeant operation comprises subjecting the propellant to cycling through a temperature range which includes a phase transition temperature of the ammonium nitrate. A temperature on either side of a phase transition temperature is chosen, and a cycle is completed when the propellant has passed from one ammonium nitrate transition phase to the next transition phase and subsequently back to the original transition phase. The conditioned propellant will undergo substantially less volume change during thermal cycling through a temperature range which includes an ammonium nitrate transition temperature, than would occur when cycling over the same temperature t me Patented Jan. 8, 1963 range prior to the specific conditioning steps. With this conditioning process, an ammonium nitrate-containing propellant can meet the Air Force specification which requires that a propellant must be able to withstand Shock cycling between -65 F. and 169 F. without cracking. A propellant grain which passes such a test can withstand the extreme temperature fluctuations such as would be encountered in flight from comparatively low to very high altitudes, transfer to arctic latitudes, etc.

The use of ammonium nitrate as the sole oxidizer in a rubber-base composite propellant is advantageous from the standpoint of the low oxidizer cost and ease of availability. This oxidizer however possesses the disadvantage of having a number of crystalline transition temperatures at which there are changes in specific volume and density. Within the Air Force shock cycling temperature range, there are two crystalline transition temperatures, e.g., 0 F. and 91 F. At 0' F. this crystalline transition is accompanied by a 2.8 percent specific volume change and at 91 F. there is a specific volume change of 3.6 percent. Such volume changes are transmitted to the rubber binder and as a result, the grain can crack and become unsuitable for firing.

The situation is, of course, aggravated in the case of grains with star shaped perforations and other grains with complex geometrical design. Case bonded grains would be particularly adversely affected by such volume changes, since adherence of the binder to the oxidizer could cause the binder to pull away from the case. The coeilicient of expansion of the metal case is so much smaller than the volume changes of the propellant composition when crossing the transition temperature of the nitrate that stresses would appear in the propellant and if the prope1 lant adheres to the case, the propellant would develop fissures. The complex grain and the case bonded grain are by no means the only propellants which are affected in this manner, but they do represent the severe cases which require a solution of this volume change problem.

It has been suggested that ammonium sulfate or ammonium phosphate be mixed with ammonium nitrate in order to prevent disintegration of the ammonium nitrate during thermal cycling. One solution proposed to minimize such volume changes is to mix about 10 percent potassium nitrate with the ammonium nitrate. However, the addition of extraneous materials such as these, tends to reduce the specific impulse of the propellant and in some cases increases the smoke formed when the propellant burns.

The process of this invention reduces the volume changes of ammonium nitrate propellants during shock thermal cycling by first cycling the propellant, generally prior to rocket assembly, usually under less severe conditions, but through a phase tr-ansiiion temperature of the oxidant. The less severe conditions can be made by cycling slowly (low temperature gradient) or by limiting the temperature range to a modest range on either side of the phase transition temperature chosen. Any temperature range which includes a phase transition temperature can be used. X-ray examination is a convenient method for determining the phase of the ammonium nitrate and hence, the treatment conditions. One method of operation is to prepare a batch or blend of batches of propellant and fabricate into grains. One or more test grains are then cycled, as later discussed. For a given batch or blend of batches operating conditions can then be established. Such a procedure is necessary since different propellant formulations and, indeed, two batches made separately but using the same formulation may vary considerably in their behavior to this conditioning process. Such pecularities of propellant formulations are well recognized by those skilled in working with such compositions.

A possible explanation of the phenomenon occurring during the conditioning or pretreatment is herein discussed; however, this explanation is not to be construed as introducing any limitations into the specification or claims. It is believed that as the particles of ammonium nitrate expand and contract, when going through a complete temperature range cycle, they Will cause a pocket to be formed inside the binder. After cycling according to this invention, the binder will soon become disengaged from around the particle of oxidizer, and the pocket, in which lies the oxidizer, will have increased in size because of the expansion of the nitrate until it is substantially as large as the maximum size to which the nitrate particle expands during cycling over the specified temperature range. Subsequent shock cycling of the propellant grain will merely cause expansion or contraction of the oxidizer within the previously formed pocket and no substantial stresses will be transmitted to the binder.

If a temperature range including the 91 F. transition temperature of NH NO is chosen for the pretreatment cycling, a temperature cycle from 70 F. to 170 F. can be used or a range as small as 80 F. to 110 F. The extent of phase transition (conversion) at the high or low temperature can be followed by X-ray analysis. This invention is mot effective when essentially complete conversion of the nitrate from one phase to the other occurs during each half cycle. This can involve a longer time than it is desirable to devote to this, so a shorter cycle time can be used with a corresponding decrease in percent phase change effected and some loss of eflectiveness of this invention. The conversion used can be in the range of 50 to 100 percent, preferably 80 to 100 percent. The time or frequency or rate of temperature change can vary over a broad range so long as a percentage conversion from one phase to another within the stated range is effected. Times in the range 1 to 500 hours per cycle give good results. However, times outside this range can be used.

When desired, the advantages of our invention are particularly apparent when a small amount of a polar material such as water, ethanol, ethyl methyl ketone, Duomeen C diacetate and the like are used. The amount of polar material added can be in the range of 0.03 to 0.3 Weight percent based on the total propellant. This polar material can be added at any time, such as to the masterbatch (copolymer and black) or to the propellant batch just prior to and during the addition of the ammonium nitrate. The latter procedure appears more effective in reducing the volume changes during thermal cycling but it is preferable to add the polar material to the masterbatch, since this maintains the physical properties of the propellant at a desirable level.

This cycling is continued until minimum specifications can be met. change obtained during the last half cycle is from about 15 to about 70 percent of the volume change of the first half cycle. It will be apparent that this reduction cannot go to zero since the oxidizer and binder possess a measurable coeificient of thermal expansion. This invention does not alter the coeflicients of thermal expansion of the propellant components, but it does reduce the volume change of the propellant caused by the crystalline transitions of the oxidizer. The latter change can theoretically be reduced to zero which would represent the ultimate of this cycling treatment.

The rubbers used for the binder of this invention are natural or synthetic rubbers with Mooney values (ML-4) generally ranging from about 5 to about 100, or higher, preferably in the range from about to about 40. Natural rubbers are well known in the rubber art. The synthetic rubbers can be prepared in any manner known in the art such as mass or emulsion polymerization. One suitable method is the emulsion polymerization of conju- Duomeen C diacetate can be defined as the diacetate of the monoamide of 1,3-diaminopropane and coconut oil.

This will generally be when the volume gated diolefins with other copolymerizable monomers at O to 140 F. in such systems as the iron-pyrophosphate, either sugar-free or containing sugar, sulfoxylate and the persulfate recipes. Any suitable emulsifier such as a fatty or rosin acid soap or the like can be used. These recipes usually contain 1 to 9 parts by weight of emulsifier per 100 parts of monomers.

The conjugated dienes which can be employed are those containing 4 to 8 carbon atoms per molecule such as 1,3- butadiene, isoprene, piperylene, methylpentadiene, 2,3- dimethyl 1,3 -'butadiene, chloroprene, 2,3-dimethyl-1,3- hexadiene, and others. However, conjugated dienes of more than 8 carbon atoms can be used, such as 2,3-diethyl- 1,3-octadiene, and the like. The various alkoxy and cyano derivatives such as 2-methoxy-3-ethylbutadiene, 2-ethoxy- 3-ethyl-1,3-hexadiene, 2-cyano-1,3-butadiene and the like are applicable. The monomer to be copolymerized with the above diene can be any monomer containing an active CH2=C group such as aryl olefins, esters of acrylic and substituted acrylic acids, nitriles, amides, ketones, and vinylpyridines. Examples of such'monomers include, among others, styrene, alpha-methylstyrene, alkyl-substituted styrenes, p-

chlorostyrene, p-rnethoxystyrene, acrylonitrile, methacryl onitrile, methyl methacrylate, butyl methacrylate, methacrylamide, methyl isopropenyl ketone, 2-vinylpyridine, 2- methyl-S-vinylpyridine, 3-ethyl-5-vinylpyridine and the like.

In the preparation of the copolymers, the amount of conjugated diene used is generally in the range of 50 to 99 parts by weight per 100 parts of monomers. Homopolymers, such as polymerized conjugated dienes, i.e., polybutadiene, are also applicable as the rubber component in the binder in our invention providing that the homopolymers are rubbery, that is, possess a Mooney value (ML-4) in the range from about 5 to about 100, or higher, and preferably from about 10 to about 40.

The propellant can have 50 to parts by weight of oxidizer and 50 to 10 parts by weight of binder per parts by weight of oxidizer-binder blend. There can be present any reinforcing agent, plasticizer, antioxidant, etc. Some reinforcing agents which can be used are carbon black, wood flour, lignin, and resins such as the styreneacrylic acid-divinylbenzene polymers.

Any antioxidant such as phenyl-beta-naphthylamine, tris-nonyl-phenyl-phosphite, or the like can be used. Any vulcanization accelerator can also be used, such as the dithiocarbamates, e.g., N,N,-dimethyl-S-tert-butylsulfenyl dithiocarbamate. When using a vinylpyridine rubber, the binder can be cured by such systems as quaternization to reduce creep under stress.

The ingredients are mixed on a roll mill or internal mixer such as a Banbury or a Baker-Perkins dispersion blade mixer. The binder forms the continuous phase in the propellant. The rocket grains are formed by com pression molding, injection molding, or extrusion. The grains are then cured by temperatures in the range of 70 to 250 F., preferably between and 180 F. The time for cure is generally around three hours with the higher temperatures to seven days for the lower temperatures. If the thermal cycling treatment of this invention is extended for a sufiicient period of time it can serve as the cure treatment also.

The following example is intended to exemplify one embodiment of our invention.

EXAMPLE I The propellant used in the following tests contained 82.5 parts of finely ground ammonium nitrate, 17.5 parts of a binder comprising a ZO-Mooney (ML-4) 90/10 butadiene/Z-methyl-S-vinylpyridine copolymer with its compounding ingredients, and two parts of Milori blue as the combustion catalyst.

5 The rubbery polymer was prepared from the following recipe:

TABLE I Rubber Polymerization Recipe 1 6 to sub-sieve size and having a geometric average size of about 40 microns; (3) mixing the binder, oxidizer, and catalyst in a 2% gallon dispersion blade mixer; and (4) molding the plastic mixture into 3-inch, solid P t b ht 5 cylindrical grains in a compression mold at about 5,000 omponen ars p.s.i.g. pressure. All these operations were conducted Water. in an atmosphere controlled to a relative humidity of 2 32:3 y py 27 percent or less. The cure time was 24 hours at 170 F. Potassium oleate 6 The ropellarit grains were cut into two unequal parts sodmm Salt of alkyl aryl Sulfomc and l0 and stofed together in a room held to a relative humidity Kcl below 25 percent until they reached room temperature. K4P2O7 Measurements, at room temperature, of the large portion 'FeSO4'7H2O of the grain were then made at three positions on the hydroperohlde diameter and one on the length. The positions of these y 232 mercaptan "averaga" measurements were marked for reproducibility. The seii i l g-naphthylamine:liased 6 two pieces were then placed together in triple polylTh bb bt d f M d f 7 r us n which etohylene 7ba gs corlgtaniqing slic? gel l incllf cyclled between from tg oviii inii gr t i i ai 'yd dec yl me i cap tan was 7 and 1 0 F' t en 0 a cyc measureemployed, Le" average 056 part l ments were made between thepieviously marked spots i fi /fi 55 3 2 dsllIigtyldlfhwcarbamate and on the ma or part of the grain. Samples were taken m e u p u for X-ray analysis from the smaller piece of propellant. t f p lil t :5 4 l ggu s 'fgfge 23765:; f The pliaseuconvelrsion was carried to 9%};100 perc'en; ll'lue 3 average lme or neary at eye es on t ese grains. e extent 0 P Conversion was 61 Percent The emulsion P Y conversion was determined by X-ray measurements using erialtiorijreagtiong piga f g w l 1;; and r em a Gieger (riourlilteii1 dillifractgmgter. A sample fhizlllder was 6 11 a 16116 Y- y PY 1 e 1 6 C constructe w ie a owe t e temperature 0 e speci- Positioll P p in accordance with the following men to be controlled by circulation of water for temf rmulailoni peratures up to 190 F. and of heptane for temperatures Component: TABLE II Parts by Weight down to -60 F. The temperature of the sample itself copolymer (butadiene/2 methy1 5 viny1pyri was measured by a thermocouple attached to the sample dine) 100 holder. The opening in the X-ray scattershield was Furnace 55 20 covered by a sheet of cellophane so that the atmosphere Benzophenone 1O surrounding the sample could be dried by a packet of A Di hen 1 1O desiccant mounted in the end of the shield. The temperi p y 6 ature of the propellant sample during testing is main- Eplchlorfhydrm 2 tained at that temperature which the sample possessed g 8 175 just prior to removal from the cycling treatment to the P ur testing operation. Zmc oxlde "5 3 40 The X-ray observations were made using a copper Aerosol. anode tube operated at 40 kilovolt peak and 18 milliam- Flexamme peres. The divergence slit was 1 and the collecting slit i3 3 gig gi gg 'fgi f fggl g gggg gg fg a was 0.003 inch. A nickel filter was used before the 3.Ph i: 1 mixtupe contming 65% 5; complgx diary} Geiger tube. In order to select regions to be used in g p r g l gg g g product and 0f N,N u identifications, a careful measurement of the X-ray patterns of three of the phases of ammonium nitrate were The propellant composltlon was as follows: made. The amount of a given phase present in the sam- TABLE III ples was assumed to be measured by the area under its Component: Parts by Weight characteristic line relative to the area under this same Amm m nitrate line for a similar sample assumed to be completely in Bin r co posi i this phase. Accuracy of detectability of a minor phase Mil ri bl 1 was about 5 percent. 1A i ment similar to Prussian blue, prepared by the As can be seen from examination of the results preglrlrlilittglon of a paste of potassium ferrocyanide and ferrous 5r id i T bl IV, a f h koi dig a to t is invention can re uce y 2 to 6 percent t e I Processing the propellant used in the tests consisted Change in propellant volume caused by thermal cycling the follQwlng p -f Vacuum drylng ammomum In some samples. water was deliberately added to the promtrate FY1115 and M f blue P Y elevated pellant during mixing. The percentage of water is based temperature; (2) grinding the dried pulls in a pulveron the weight of propellant. All grains were judged good izer at 14,000 r.p.m. to particles ranging from 100 mesh for firing after cycling.

TABLE IV Run No i 2 3 4 5 6 7 Temp. of molding room, 91

F above below above above above below above Water added based on total propellant, wt. percent..." none none 0.15 0.1 0.1 0.1 none Volume change for half of first cycle, percent 3.1 3. 5 3.1 2. 7 3. 7 2.6 3.0 Volume change for half of last cycle, percent 2. 3 2. 2 0.8 1. 5 1. 5 1. 5 1. 8 Number of half cycles used 11 12 10 11 11 11 11 Total cycling time, hours. 770 670 685 550 560 405 600 1 Added before all the nitrate was incorporated. a

It will be apparent to those skilled in the art that various modifications and applications of the invention can be made upon study of the accompanying disclosure Without departing from the spirit and scope of said disclosure. While the invention has been described in connection with ammonium nitrate, it is also applicable to any other solid material which is useful as a propellant component and which undergoes a pseudo change of state or a transition from one solid phase to another, the change or transition being accompanied by a change in specific volume.

We claim:

1. A method for increasing the resistance to thermal change of a propellant composition comprising ammonium nitrate and a rubbery binder, which method comprises repeatedly varying the temperature of said composition through a cycle ranging from below a phase transition temperature of ammonium nitrate, at which temperature the ammonium nitrate undergoes a phase transition from one solid phase to a different solid phase, to above said transition temperature and again to below said transition temperature, While maintaining said com position essentially in the solid state, and continuing the described temperature cycling until the volume change of said composition during a half cycle is from about 15 to 70 percent of that during the first half cycle, wherein the 0.1 to 0.3 Weight percent of a polar compound selected from the group consisting of Water, ethanol, methyl ethyl ketone, and the diacetate of the monoarnide of 1,3-diaminopropane and coconut oil is added to said propellant.

2. The product produced by the process of claim 1.

References Cited in the file of this patent UNITED STATES PATENTS 2,570,632 Barton Oct. 9, 1951 2,590,054 Taylor et al. Mar. 18, 1952 2,657,977 Stengel et al. Nov. 3, 1953 2,742,672 Thomas Apr. 24, 1956 2,877,504 FOX Mar. 17, 1959 2,930,683 Adelman Mar. 29, 1960 2,938,778 Linsk May 31, 1960 FOREIGN PATENTS 655,585 Great Britain July 25, 1951 

1. A METHOD FOR INCREASING THE RESISTANCE TO THERMAL CHANGE OF A PROPELLANT COMPOSITION COMPRISING AMMONIUM NITRATE AND A RUBBERY BINDER, WHICH METHOD COMPRISES REPEATEDLY VARYING THE TEMPERATURE OF SAID COMPOSITION THROUGH A CYCLE RANGING FROM BELOW A PHASE TRANSITION TEMPERATURE OF AMMONIUM NITRSTE, AT WHICH TEMPERATURE THE AMMONIUM NITRATE, AT WHICH TRANSITION FROM ONE SOLID PHASE TO A DIFFERENT SOLID PHASE, TO ABOVE SAID TRANSITION TEMPERATURE AND AGAIN TO BELOW THE 0.1 TO 0.3 WIEGHT PERCENT OF A POLAR COMPOUND SEPOSITION ESSENTIALLY IN THE SOLID STATE, AND CONTINUING THE DESCRIBED TEMPERATURE CYCLING UNTIL THE VOLUME CHANGE OF SAID COMPOSITION DURING A HALF CYCLE IS FROM ABOUT 15 TO 70 PERCENT OF THAT DURING THE FIRST HALF CYCLE, WHEREIN THE 0.1 TO 0.3 WEIGHT PERCENT OF A POLAR COMPOUND SELECTED FROM THE GROUP CONSISTING OF WATER, EHTANOL, METHYL ETHYL KETONE, AND THE DIACETATE OF THE MONOAMIDE OF 1,3-DIAMINOPROPANE AND COCONUT OIL IS ADDED TO SAID PROPELLANT. 